About 15 years ago, after the Playstation release, "modchips" started to surface. Theses chips allowed to use imported or copied games. The first release were using a PIC 16C54. A one time programmable microcontroller. This chip sends specific data stream to the main CPU, in order to authenticate the CD (SCEE / SCEI / SCEA, according to region). Then, the PIC was analysed and reversed by "Old Crow", and was called "REI Version". The code was ported to 16C84 and 16C54. In 1997, the PIC 12C508 was released by Microchip. Thanks to its new features (internal oscillator for example), this PIC became the best choice for a mochip, and the REI code was ported on it. This code saw multiple revision, to follow the various Playstation hardware versions. (source)

Then, with Final Fantasy 8, Sony released a protection against modchips, which printed a warning message when the game was running on a modchipped console. But, a few weeks after, the REI code saw a new revision to support "Stealth" mode (by Mr Bongo, an active member of the Playstation scene).

Sony didn't stop here : with the Dino Crisis release appeared the V.0 of whan would become libcrypt, their new weapon against copy. This time, even "Stealth" modchips were detected. The first solution was to start console in the "Dino" mode, using reset button, but then, new modchips arrived, like MultiMode and Onechip (the latter was only compatible with Psone). Today, these are the two sold modchips if you want to "chip" your Playstation.

A couple years ago, GaryOPA, a former psx-scene admin and actual manager of PS3crunch adapted the code of the first REI version, for the PIC 12F629. This pic became famous thanks to the Wii scene, and has a huge advantage over the 12C508 : it can be reprogrammed. And it is cheaper.

Recently, "someone" (Tulchan Gamps) gave me the reversed source of MultiMode 3, adapted for 12F629. In the archive, there are 3 versions, each for every Playstation region. I could only test for European Playstation, with import and copied games. As I don't have any libcrypt protected game, I couldn't check if the modchip was fully working.

The installation diagrams are the same as for the 12C508 version, and easily available on Internet.